1. Field of the Invention
The present invention generally relates to electronic voltage controlled oscillator (VCO) circuits having particular application in phase locked loop circuits and, more particularly, to a fully differential, fully symmetrical VCO circuit with low susceptibility to power supply and current source noise.
2. Description of the Prior Art
Phase locked loops are used in many diverse applications, including optical serial data links, multiple clock frequency generation, and FM demodulation just to name a few. An important component of a phase locked loop is the voltage controlled oscillator (VCO). It is desirable to integrate all of the components of the VCO onto a single silicon chip for cost minimization as well as noise considerations.
U.S. Pat. No. 4,833,425 to Culican et al. provides just such an integrated VCO. However, when operating in the range of hundreds of MHz and beyond, it is extremely important to minimize the effects of noise on the power supplies and at inputs, so a differential approach is needed. M. N. Shen et al., in "High-Frequency Voltage-Controlled Oscillator", IBM Technical Disclosure Bulletin, vol. 33, no. 5, October 1990, pp. 260-262, describes a monolithic high frequency voltage controlled oscillator consisting of a temperature and voltage-compensated control voltage generator and a ring type voltage-controlled oscillator. The ring type voltage-controlled oscillator is composed of N stages of differential amplifiers with the outputs of the Nth stage being crosscoupled with the inputs of the first stage. Oscillation frequency is determined by the number of stages N and the bias current of the individual differential amplifiers. While the Shen et al. circuit uses differential amplifiers, the control voltage is single-ended.